1. Field of the Invention
The present invention relates to axial flow turbines incorporating an exhaust hood and more particularly to arrangements for improving the flow and pressure distribution in the exhaust from such turbines.
2. Description of the Prior Art
In the discharge of exhaust steam from an axial flow turbine, for example discharge of this exhaust steam to a condenser, it is desirable to provide as smooth a flow of steam as possible and to minimize energy losses from accumulation of vortices and turbulences and non-uniformity in such flow.
Usually the exhaust from the turbine is directed into an exhaust hood and from there to a condenser. It is desirable to achieve a smooth transition from axial flow at the exhaust of the turbine to radial flow in the exhaust hood and thence a smooth flow at the discharge opening of this hood into the condenser.
In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a relatively uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected.
It is also desirable to achieve optimum efficiency at the last stage buckets of the turbine prior to exhaust from the turbine by achieving a relatively uniform circumferential and radial pressure distribution in the exit plane of the last stage buckets. Finally, it is desirable to accomplish these results while employing a hood having as short an axial length as possible.
The prior art has employed, in the exhaust duct connected to the turbine, vanes which have smoothly curved surfaces for effectively changing the axial flow of the steam from the turbine to the generally radial flow. For example of such an arrangement for converting the axial flow of the exhaust from the turbine to radial flow is shown in U.S. Pat. last 3,552,877 - Christ et al.
Such arrangements, however have not provided for effectively directing of the exhaust steam to the discharge opening of an exhaust hood with reduced acceleration losses and reduced losses resulting from the forming of energy-consuming vortices in the flow of the exhaust steam. Moreover, they have not effectively achieved relatively uniform circumferential and radial pressure distribution at the exit plane of the last stage turbine buckets, a consideration which is of increasing importance for buckets having high tip speeds and high exit Mach numbers.
By the present invention additional guiding and diffusing vanes are employed to effect a smooth transition from radial flow to a direction leading to the discharge opening of the exhaust hood. It has been found that the arrangement of the present invention results in a substantially uniform flow thru across the entire area of discharge opening of the exhaust hood. Moreover, the guidance of the steam flow by the arrangement of this invention has been found to minimize the generation of energy-consuming vortices and to minimize acceleration losses in the guide vanes employed in the structure of this invention. In addition, a relatively uniform pressure distribution in the exit plane of the last stage buckets is obtained. These improved results have been obtained while still employing an exhaust hood having a relatively short axial length.
Accordingly it is an object of this invention or provide an axial flow turbine incorporation an exhaust hood which achieves a substantially uniform flow distribution at the discharge opening of the exhaust hood.
It is another object of this invention to provide an axial flow turbine incorporating an exhaust flow in which acceleration losses in the fluid passing thru the hood are minimized.
It is a further object of this invention to provide an axial flow turbine incorporating an exhaust hood wherein the generation of energy consuming vortices in the flow of fluid thru the hood is minimized.
It is still a further object of this invention to provide an axial flow turbine incorporating an exhaust hood wherein a relatively uniform circumferential and radial pressure distribution in the exit plane of last stage buckets of the turbine is obtained.